Noise eliminating circuit for a graphical input terminal

Abstract

A noise eliminating circuit is provided for a graphical input terminal having an input unit which generates electrical signals when a stylus is used to write on a surface of the unit. The electrical signals correspond to information written on the surface. Further, an envelope signal is generated responsive to the starting of the writing, which continues throughout the writing, and ends at the time when the writing stops on the surface. A low-pass filter responds to the electrical signals representing the written message to remove high frequency noise therefrom. A switch selects either the electrical signals or the output of the low-pass filter as the output signal to the noise eliminating circuit. The switch is operated in response to the envelope signal specifying the starting and stopping times.

I claim: 
     
       1. A noise eliminating circuit for a graphical input terminal having input means including a hand-operated stylus to write on an input surface, said input means generating an electrical indication corresponding to information written on said input surface by said hand-operated stylus, said input means also generating a first signal including information specifying at least a starting time of a writing operation on said surface, said noise eliminating circuit comprising: low-pass filter means coupled to receive said electrical indication; switch means for selecting either said electrical indication or the output signal from said low-pass filter means as an output signal of said noise eliminating circuit; control means which is responsive to the first signal including said starting time-specifying information to control said switch means for continuing said selection for a predetermined duration of time after said starting time; and means for changing said signal including the time-specifying information to a predetermined level only during the writing operation. 
     
     
       2. A noise eliminating circuit for a graphical input terminal as claimed in claim 1, in which said control means includes a monostable multivibrator triggered by said signal including said time-specifying information at said starting time to produce a pulse for said predetermined duration of time, and means including an exclusive OR gate circuit operating between said signal including said time point-specifying information and the output of said monostable multivibrator. 
     
     
       3. A noise eliminating circuit for a graphical input terminal as claimed in claim 1, in which said control means includes a first monostable multivibrator triggered by said signal including said time-specifying information at said starting time, said first multivibrator producing an output pulse for a first predetermined duration of time, said signal from said input means also indicating a terminating time of said writing operation, and a second monostable multivibrator triggered by said signal including said terminating time of said writing operation, said second multivibrator producing an output pulse for a second predetermined duration of time. 
     
     
       4. A noise eliminating circuit for a graphical input terminal having input means including a hand-operated stylus to write on an input surface, said input terminal giving a first signal which includes information specifying at least a starting time of a writing operation and thereafter continues as long as said stylus engages said surface so that said given first signal includes both said starting time-specifying information and also the terminating time of said writing operation, said stylus generating an electrical indication corresponding to information written on said input surface by said hand-operated stylus, said noise eliminating circuit comprising: low-pass filter means coupled to receive said electrical indication and to give an output signal; switch means for selecting either said electrical indication or the output signal from said low-pass filter means as an output signal of said noise eliminating circuit; control means which is responsive to the first signal including said starting time-specifying information to control said switch means for continuing said selection for a predetermined duration of time after said starting time; said control means being operated responsive to said given first signal including said starting time-specifying information to select said indication for a first predetermined duration of time after said starting time and a second predetermined duration of time after said terminating time; said control means further including a first monostable multivibrator triggered by said signal including said time-specifying information at said starting time, said first multivibrator producing a pulse for said first predetermined duration of time, and a second monostable multivibrator triggered by said signal including the time-specifying information at said terminating time, said second multivibrator producing a pulse for said second predetermined duration of time.